1. Field of the Invention
This invention relates to a thermosetting resin composition and more particularly it relates to a thermosetting resin composition which will exhibit not only good mechanical properties, electrical properties and a good water resistance but also an improved crack resistance when used as an impervious encapsulation material of an electronic component.
2. Description of the Prior Art
Heat dissipation has recently presented a serious problem with the increase of degree of integration in a resin-encapsulated electronic component. Reduction in the thickness and size of electronic parts has intensively been effected. Furthermore, practical service conditions of the parts have become increasingly severe, resulting in frequent occurrence of peeling of an encapsulating material from an encapsulated member and cracking of the encapsulating material itself due to repeated heat cycles during the use of the parts. This may sometimes lead to deteriorate the electronic parts.
As a countermeasure against these problems, there has heretofore been employed, for example, a method in which a large amount of an inorganic filler is incorporated in a resin to approximate the volumetric coefficient of thermal expansion of an encapsulating material to that of a member to be encapsulated therewith. However, with an increase in the amount of an inorganic filler blended, it is increasingly likely not only that the inorganic filler might damage the surface of the encapsulated member, but also that the workability of the filler-blended encapsulating material is lowered disadvantageously because of the increased viscosity, thus presenting a big difficulty in handling particularly in the case of use of said blended encapsulating material as a liquid encapsulation material.
As another countermeasure, there has been an attempt to blend a flexibilizer (that is, a flexibility-providing agent) with a resin. A glycidyl ether derived from a polyalcohol such as 1,4-butanediol, poly(alkylene oxide)glycols, or glycerin is wellknown as a flexibilizer. However, the reactive flexibilizers of this kind cannot make a sufficient expected improvement in the flexibility of the resin and, what is worse, it is defective in that they markedly lower the electric properties, heat resistance, and water resistance of the encapsulated material.
On the other hand, it is known that a butadiene polymer, an epoxidized butadiene polymer or a butadiene polymer-modified epoxy resin can be used to improve the crack resistance of an encapsulated material. When a butadiene polymer is used as such an improver, it exhibits poor compatibility with an epoxy resin so that it cannot be homogeneously blended with the epoxy resin or it moves to the surface of a molding in the course of molding. This spoils the appearance of the molding, leading to reduction in the commercial value.
When an epoxidized butadiene polymer is used, a uniform cured product disadvantageously cannot be obtained since the reactivity of the epoxy groups of epoxidized butadiene polymer is notably lower than those of the epoxy groups of a bisphenol A type epoxy resin and a novolak type epoxy resin.
The butadiene polymer-modified epoxy resin is prepared by reacting a butadiene polymer having carboxyl groups at its terminals with an epoxy resin. In the preparation, alcoholic hydroxyl groups formed in a reaction of carboxyl groups with epoxy groups are further reacted with epoxy groups, thus presenting a problem that a final product is liable to have a three-dimensional network structure which make the molding composition difficult to cause to flow and to fill the mold satisfactorily. Another disadvantage is that the butadiene-modified epoxy resins are expensive.